Abstract A method for measuring isotope ratios of plutonium and uranium in plutonium or uranium–plutonium (Pu/U) mixture samples was developed by using a continuous heating method of thermal ionization mass spectrometry, which was previously shown to be effective in measuring isotope ratios of trace uranium. In this method, temperature of an evaporation filament is increased sequentially. This leads to measure the ratios of Pu and U at different temperatures in which each element is evaporated efficiently. Under optimal measurement conditions, the intensity of 239 Pu and the ratio of 240 Pu/ 239 Pu were measured down to sample amounts of 0.1 fg and 0.3 fg, respectively. In Pu/U mixture samples with pg-level masses and 0.01–10 Pu/U ratios, isotope ratios of 240 Pu/ 239 Pu and 235 U/ 238 U were measured simultaneously without chemical separation of samples. The relative standard deviation of the isotope ratios of 240 Pu/ 239 Pu and 235 U/ 238 U in the sample containing each 1 pg of plutonium and uranium were better than 2% and 4%, respectively. Moreover, 238 Pu/ 239 Pu ratios were obtained by using a correction in which the 238 U intensity was estimated from the 235 U intensity and the 235 U/ 238 U ratio. The results in this study suggest that the continuous heating method of thermal ionization mass spectrometry will be effective tool as a method for measuring isotope ratios in the Pu/U mixture samples.